Just How Toxic Are Carbon Nanotubes?

Carbon nanotubes (CNTs) add strength and design flexibility to many materials, from reinforcing adhesives and materials used in electronics to enhancing the design of carbon fiber composites for use in cars and aerospace applications. But there's been increasing evidence that they, along with other nanoscale materials, may pose risks to human health and the environment. In ongoing research efforts to determine just how toxic they actually are, researchers at Texas Tech University have come up with a new method for detecting CNTs in soils.

The university's environmental biologists built an apparatus that can quantify how much CNT is present in a given soil sample. That's not an easy task since they're so small: mean outer diameters of 13nm to 16nm are common in multi-walled tubes. The apparatus was developed as part of the researchers' ongoing work in locating CNTs in biological environments and examining how they to accumulate in soil, plants such as food crops, or other organisms.

Researchers at Texas Tech University have come up with a new method for detecting CNTs in soils, which will help determine their toxicity. CNTs are so small that mean outer diameters of 13nm to 16nm are common in multi-walled tubes, shown here as grains partially smeared on paper (scale in centimeters). (Source: Shaddack/Wikimedia Commons)

The testing apparatus works by exposing soil samples to microwaves, which can reveal even mere trace quantities of CNTs. That's because, in the presence of microwaves, CNTs produce very high quantities of heat, much higher than most materials. The effect is so extreme that, if you put CNTs in your kitchen microwave oven, the carbon will spontaneously ignite. The researchers used this fact to heat samples to different temperatures to determine different concentration levels. The method has also been used to determine the amount of CNT loading in plant samples and earthworms.

The news about CNTs' toxicity is more than a little unwelcome, since graphene, the tubes' constituent substance, is the hardest known substance. When layers of it are rolled up into tubes, the resulting CNTs constitute a fiber that is 100 times stronger than steel and weighs one-sixth as much. That combination, along with qualities such as mechanical strength and electrical and thermal conductivity, makes them highly unusual and ideal building blocks for industrial uses.

Earlier this year, a joint study by the University of Missouri and the US Geological Survey showed CNTs to be toxic to various species of invertebrate aquatic organisms that live in sediment, including mussels, worms, and crustaceans. These researchers note that CNTs, which may contain metals as well as carbon, tend to accumulate in sediment when released into water. Both the metals -- including nickel, chromium, and other metals used in manufacturing that may remain as impurities -- and the carbon in CNTs can reduce growth rates or even kill certain types of marine life. The degree of toxicity varied depending on the type and source of CNT, the species of test organism, whether the materials had been cleaned using acid, and what method was used for dispersing the materials.

Researchers of the joint study say one of the biggest potential contamination risks occurs during the manufacture of carbon composites, but careful waste management and handling procedures can reduce that risk. More information is also needed on what happens when the composites begin to break down.

@Cabe: It's definitely an exaggeration to say that everybody who worked at the Johns Manville plant in Waukegan died of asbestos related problems -- but many people did, and not just people who worked in the plant. The company was careless about what it did with waste material (it was literally just strewn around the site), so there were significant amounts of airborne asbestos dust in the surrounding area. This was well after asbestos was known to be harmful.

On the other hand, a company I worked for in Chicago went bankrupt in 2002 as a result of an asbestos class-action suit. It was one of the small companies that lawyers went after once all of the big companies like Johns Manville went under. As far as I know, the company's use of asbestos had always been relatively minor, and ended in 1972, when the dangers of asebstos began to be widely known. But a few lawyers, including one who had a (now-defunct) website called MillionDollarLungs.com, saw dollar signs.

Asbestos continues to be widely used in the developing world, particularly in India, where the use of asbestos is actually expanding.

As George Santayana said, "Those who cannot remember the past are condemned to repeat it."

It is a shame what past generations left us, like these contaminated factories. Could they have known? Those 4000+ people probably all suffer from asbestos related problems. Could they have known in the past.

I agree with you all. There is no excuse for not testing for human safety in these new fabrication materials. Perhaps 4000+ people's lives will be saved with that work.

Dave, thanks for the info and links, and the perspective of one directly affected by environmental toxicity. Cabe, the toxicity potential is far, far worse with nanomaterials than with materials that have micro-sized particles, such as silicon. It's a matter of scale, for one thing: in this case, size matters enormously (pun intended). You might want to take a look at some of the background material, such as the links Dave provided or those in my previous nanomaterial legislation article, to understand how different events at the nanoscale can be from events at the microscale. It's an eye-opener.

@Cabe: Obviously everything has risks, but it's important to understand the risks, so that you can keep them under control. There are three Superfund sites within walking distance of my house. One is a former Johns Manville insulation plant that once employed 6500 people. It is still contaminated with more than 3 million cubic yards of asbestos. Another is our harbor, which is contaminated with PCBs that were used in a former die casting facility that employed more than 4000 people. Not only are the jobs gone, but we're left with the mess and its long-term effects. This is why it's important to understand the health and environmental implications of a material before employing it on a mass scale.

Great, one of the future super-materials is poised to kill us all. I suppose silicon has been killing life for decades directly and indirectly. What else is new. Perhaps the study should look at how the nano-tube compares to the material it is replacing. I'm sure as materials like this become commercialized, they will.

Tim and ervin0072002, based on the Japanese, US and European concerns about CNTs from all sources, which we covered herehttp://www.designnews.com/document.asp?doc_id=237995and especially during the manufacturing process, I think the answer to Tim's question is "yes."

@Ann: You're definitely right that environmental and health impacts should be at the front of our minds when evaluating a new material. That being said, there has been a lot of work done on the toxicity of nanomaterials, particularly over the past 5-7 years. A lot of people are working on this, to ensure that we aren't opening Pandora's box.

A German research group published an article in Nature in 2006 showing that a Damascus blade produced in the 17th century contained multi-walled carbon nanotubes, as well as cementite nanowires. It's believed that the nanotubes formed in-situ during the forging process. You can read the article here.

The article mentions that cobalt, along with other alloying elements present in small amounts, played an important role in providing the steel with its distinctive microstructure.

Lou, thanks for your comments. Researching and writing this article made me think how, for the nth time, we've gone off looking for new technologies without first considering whether the (new or old) materials involved are harmful to living beings when introduced into the ecosystem, or even how likely it is that the materials can easily get into the ecosystem. It's simply not one of the first questions we ask--and I think it should be.

University of Southampton researchers have come up with a way to 3D print transparent optical fibers like those used in fiber-optic telecommunications cables, potentially boosting frequency and reducing loss.

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